The present invention relates to the manufacture of terry cloth and, in particular, to apparatus for achieving a uniform ratio of pile to the ground warp which constitutes the fabric's foundation.
For many years terry cloth was produced utilizing conventional fly shuttle looms. Such looms weave a product with a uniform pile-to-ground warp ratio, but they typically operate at a relatively slow rate. More recently, however, fly shuttle looms have been replaced with high speed weaving machines such as the Models PU and TW 11 looms produced by Sulzer Brothers Limited of Winterthur, Switzerland. Various aspects of these types of machines are the subject of several publications, including U.S. Pat. Nos. 3,871,419, 4,122,873 and 4,569,373.
In a Sulzer machine, ground and pile warps move past a reciprocally operable reed and a displaceable rocking bar. The ground warp continuously is dispensed from its supply beam, while the pile warp is dispensed incrementally from its supply beam under the control of a pile warp let-off motor. A weft or filling yarn is inserted between the reed and the movable rocking bar in the weaving operation, and as the reed are displaced towards the rocking bar during their reciprocation, the filling yarn is carried by the reed to the fell of the cloth being woven.
In a typical weaving cycle, the rocking bar is maintained at a first position as the filling yarn is carried to the fell twice in succession in the manner just described. Before the reeds are displaced a third time, however, the pile warp let-off motor dispenses pile yarn, and the rocking bar is displaced to move the fell of the cloth towards the reeds. As a result, when the reed carries the filling yarn to the fell of the cloth, loops of the pile yarn are formed in a row across the top and bottom of the base fabric. The rocking bar then is withdrawn to its initial position to permit the three-pick weaving cycle just described to be repeated.
The height of the loops in terry cloth is very important to its acceptability. In a typical high pile terry, approximately 55% of the total fabric is pile yarn. Any fluctuation in pile height (i.e., a change in the pile-to-ground warp ratio) has an adverse effect on the fabric's weight and appearance.
Two kinds of pile warp let-off can be used in a terrying operation. The first is a positive type pile let-off, a mechanically-linked device which lets-off a predetermined amount of terry yarn based on a mechanical adjustment. The second type--employed in a Sulzer machine--is a negative pile let-off motor which controls let-off in dependency on pile warp tension, the amount of terry yarn dispensed being that required to maintain constant tension on the pile warp.
Terry looms with a motorized negative-type let-off attempt to control the pile-to-warp yarn ratio by monitoring the tension of pile yarn at a location near its supply beam. More particularly in one known version of a Sulzer machine, the ends of pile yarn pass over a flexible beam as they are fed into the loom. A metallic flag is secured to the beam so as to move towards or away from the pile yarn supply beam as the beam flexes in response to the amount of tension applied to the pile warp ends. A proximity sensor is mounted adjacent the flag. This sensor produces an output voltage having a magnitude dependent on the distance between it and the flag. As tension on the pile warp ends changes, the flag's movement alters the sensor's output voltage. This output voltage is supplied to circuitry for producing signals for increasing or decreasing the speed of the pile warp let-off motor to alter the amount of pile yarn dispensed from its supply beam thus maintaining constant tension on the yarn. As the pile warp tension increases, the pile warp let-off motor accelerates so as to decrease the tension. Conversely, a lowering of pile warp tension results in the pile warp let-off motor being slowed in order that the pile warp tension will increase.
An arrangement analogous to that just described for controlling pile warp tension is described in the aforesaid U.S. Pat. No. 4,569,373.
While the arrangements just described contribute to the control of the pile-to-ground warp ratio by maintaining the tension of the pile warp within a normal operating range, the terry height nevertheless still can vary by an unacceptable amount.